1. Field of the Invention
This invention relates to the recovery of oil from subterranean formations.
2. Description of the Prior Art
Primary oil recovery methods rely on the natural energy present in oil reservoirs which forces the free oil up through wells drilled therein to the surface. After dissipation of the natural energy of the reservoir, however, there are often large amounts of oil remaining. It is common practice to utilize supplemental recovery techniques to recover the oil remaining in the reservoir. These supplemental recovery techniques are termed secondary, tertiary, etc., recovery operations depending upon their place in the series of operations carried out in the reservoir.
A commonly employed supplemental recovery operation, because of its simplicity and low cost, is water-flooding wherein a liquid, usually aqueous, is injected into the oil bearing formation through one or more injection wells drilled therein. The injected liquid forces the oil through the formation to a production well. Because oil and water are highly immiscible and due to the fact that there is high interfacial tension between aqueous liquid and oil, flooding with substantially pure water is not highly efficient. Inasmuch as secondary water-flooding recovery techniques leave a significant proportion of the oil remaining in the formation, tertiary and subsequent supplemental operations are often required to complete the recovery program.
Numerous efforts have been undertaken to reduce the interfacial tension between water and oil in water-flooding techniques to improve the efficiency thereof. For example, surfactants have been incorporated in the injected liquid to reduce interfacial tension. A wide variety of surface active agents have been employed, e.g., petroleum sulfonates (U.S. Pat. No. 3,468,377); ethylene oxide adducts of alkyl phenols (U.S. Pat. No. 3,553,130); etc.
Although increasing the efficiency of the water-flooding techniques, the use of surfactant containing fluids does not usually result in recovery of all of the oil contained in the formation in one recovery operation. Thus, liquids containing surface active agents lack the viscosity necessary to displace the oil as a flat or planar front. Thus, the surfactant solutions tend to "finger" through the oil, resulting in an incomplete recovery.
In an effort to increase the viscosity of the injected fluid, it has been proposed to substitute therefor the so-called "micellar dispersions". These generally take the form of a water-external dispersion or emulsion of a hydrocarbon, polymer, etc., formed with the aid of a surfactant. Supplemental recovery techniques based upon the use of micellar dispersions are more efficient than water-flooding operations utilizing aqueous solutions of surfactants; however, they are not without disadvantages. For example, the surfactants contained in the micellar dispersions are often adsorbed by the rock surfaces in the oil bearing formation thereby resulting in a breakdown of the emulsion as it advances through the formation.
It has been suggested to preliminarily inject into the reservoir a solution of a "sacrificial" surfactant which is adsorbed by the rock formation. This occupation of the available adsorption sites by the "sacrificial" surface active agent substantially eliminates the adsorption thereby of the surfactant in the micellar dispersion. Since the sacrificial surfactants employed heretofore are relatively inexpensive as compared to the cost of the surfactant in the micellar dispersion and inasmuch as they are highly adsorbed by rock formations in the reservoir, they greatly enhance the efficiency and cost-effectiveness of recovery operations.
It has been suggested to employ medium molecular weight sulfonated petroleum sulfonates as sacrificial agents for the high molecular weight petroleum sulfonates utilized in micellar dispersions. See Gale et al, Soc. Pet. Eng., Vol. 13, No. 4, p. 191 (1973). U.S. Pat. No. 4,006,779 discloses the use of lignosulfates as sacrificial agents.
It is an object of the present invention to provide a supplemental oil recovery method which utilizes a micellar dispersion and a sacrificial surfactant which surpasses in efficiency and cost-effectiveness those suggested and employed heretofore.